1. Field of the Invention
This invention relates to a two-stroke internal combustion engine and more particularly to an improved piston construction particularly suitable for use on a crankcase scavenged two-stroke engine.
2. Description of the Prior Art
The two-stroke internal combustion engine due to its few number of moving parts and simple rugged construction has been used as a power source in many applications. A piston is disposed within a cylinder for relative reciprocating motion. As the piston moves, it periodically covers and uncovers ports in the cylinder wall which provide releases for the exhaust gases and feed fresh fuel-air mixtures into the cylinder. In a crankcase scavenged two-stroke engine, the crankcase is sealed and functions as a pump in conjunction with the piston to feed a fresh fuel-air mixture into the cylinder. During normal operation when the piston ascends, a slight vacuum is produced in the crankcase until the lower edge of the piston skirt passes the inlet port permitting a fresh fuel-air mixture to be drawn into the sealed crankcase. A transfer passage having an opening in the crankcase and in the cylinder is utilized for moving the fresh fuel-air misture from the crankcase into the cylinder as the piston descends. With the piston in the proper position the transfer passage provides free communication between the crankcase and the cylinder. As the piston descends, the fuel-air mixture in the crankcase is slightly compressed so that when the top of the piston releases the transfer ports opening into the cylinder the fresh fuel-air mixture passes into the cylinder. As the piston descends, due to the pressure caused by the hot burnt gases, exhaust ports in the cylinder are uncovered through which the exhaust gases can pass. As the fresh fuel-air mixture enters the cylinder from the crankcase the remaining burnt gases are forced out the exhaust ports. The inlet, exhaust, and transfer ports, normally utilize a plurality of openings formed in the cylinder walls. These ports are vertically aligned and circumferentially spaced apart in the cylinder wall. Vertical continuous wall portions or ribs are formed between the circumferentially spaced ports.
Extensive work has been carried out on piston development to eliminate specific problems. U.S. Pat. No. 1,673,775 illustrates an anti-detonation piston head formation for internal combustion engines. This construction shows a plurality of holes formed in an angled portion of the cylinder skirt which are perhaps provided as drain holes for oil scraped from the cylinder wall by the sealing rings. The use of these holes is not discussed in U.S. Pat. No. 1,673,775 and they could not operate to provide a hydrostatic bearing surface as provided in the present disclosure.
Several prior art patents such as U.S. Pat. No. 1,896,124; 2,013,983; 2,151,291; and 2,151,698 illustrate piston constructions having hollow chambers formed therein through which a cooling gas or a gas to be heated is passed. The openings into the hollow chambers are relatively large and they could not function to provide the advantages of the piston described in the instant application. U.S. Pat. No. 1,953,109 discloses a piston having a sealed hollow chamber formed therein through which relatively large elongated oil collection ducts pass. The sealed chamber is provided with a cooling medium disposed therein to provide for cool operation of the piston.
U.S. Pat. No. 3,161,188 teaches a piston construction having cooling passageways formed in the piston head. The passageways are formed of inner-communicating bore holes skewed relatively to the longitudinal axis of the piston. Passages extend from the external piston skirt, through the skirt, into an opening in the inner portion of the piston and into the piston head. Oil is force squirted beneath the piston skirt into the cooling channels. The portion of the bore through the piston skirt is only formed for ease of construction. A drainage groove is provided on the external surface of the piston skirt to drain away any oil which passes through the bore of the piston skirt. The internal surface of the piston is formed to prevent or minimize any oil passing through the bore in the piston skirt.
U.S. Pat. No. 3,667,443 teaches an internal combustion engine with pistons having vent openings connecting the space between the first and second piston rings with the engine crankcase, to vent hydrocarbon-rich gases to the crankcase. A plurality of radial oil drain holes are also provided in the piston connecting the base of the third ring groove with the piston interior to provide for direct return to the crankcase of oil scraped from the cylinder walls.
A continuing problem with prior art two-stroke engines has been the generation of excessive heat in the cylinder and pistons. This limits the maximum permissible RPM's as well as reduces the volumetric efficiency of a crankcase scavenged engine. Excessive heat in the piston can also cause piston deterioration and failure of the sealing rings.